The Houston Intermediate-Voltage Electron Microscopy facility is a unique NCRR-supported National Center devoted exclusively to the development of electron crystallography for determination of biological structures at atomic resolution. Our technological development in the core research has focused on specimens that possess crystalline, helical, or icosahedral symmetry and are not amenable to study by X-ray diffraction or NMR spectroscopy. We have selected three specimens for technological research and development. The specimens are: l) the thin and highly ordered crystals of crotoxin complex from Crotalus d. terrificus venom, 2) the crystalline acrosomal bundle of helical filaments from Limulus sperm, and 3) the large icosahedral capsid particles from herpes simplex virus. High- resolution structures of these specimens will provide crucial data for understanding enzymatic neurotoxicity, actin cross-linking, and viral assembly, respectively. During the current grant period, we have made significant progress both in analyzing the structures of these specimens and in developing the technology. In this proposal, we aim to obtain three-dimensional density maps of crotoxin complex at resolutions of 3 to 5 Angstroms, of acrosomal bundle at resolutions of 7 to 9 Angstroms, and of herpes capsid particles at resolutions of 10 to 15 Angstroms. Each of these projects requires specific technological developments for specimen preparation, data collection, and computational analysis. Furthermore, we will enhance our technological infrastructure to benefit all projects. For example, the electron microscope operations will be mostly controlled by computer, data collection will be computer assisted, and a rotation cryoholder will be used to maximize the data collection efficiency. We will also evaluate the advantages of using a 1024 x 1024 slow-scan change coupled device camera for direct data acquisition. New procedures for image analysis will be developed to retrieve the high-resolution structures of the specimens under investigation. Other activity components of a NCRR Center include collaborative and user research, dissemination, teaching, training and advisory group. We propose to engage in six on-going collaborative projects and four user projects. These specimens include viruses, membrane proteins, multisubunit protein assemblies, cytoskeletal protein complex, and protein-nucleic acid complex. Many new collaborative and user/service projects will be expected to initiate in the coming years. Our criteria for choosing future projects will be their biological significance and compatibility with our technological goals. We will disseminate our new technology by means of refereed publications, seminars, and various media presentations. We plan to continue our active participation in didactic teaching and in training postdoctoral fellows and graduate, medical, undergraduate, and high school students. An active advisory committee of eight preeminent scientists will hold meetings annually to review and critique the Center's activities. We envision that the proposed research will contribute productively to the advancement of electron crystallography as a unique, reliable, and quantitative method for understanding molecular recognition and interactions central to various cellular processes.